The potential uses and recognized advantages of employing electrical energy for surgical purposes are ever-increasing. In particular, for example, electrosurgery techniques are now being widely employed to provide significant localized control advantages in both open and laparoscopic, including arthroscopic, applications relative to prior traditional surgical approaches.
Electrosurgical techniques use an instrument with working surfaces that contact tissue, such as a tissue ablation or cutting device, a source of radio frequency (RF) electrical energy, and a return path device, commonly in the form of a return electrode pad. The working surfaces that contact the patient in the region where the surgical effect is to occur are commonly called the active electrode or electrodes. The return path device contacts the patient either directly on the tissue or indirectly through, for example, a conductive liquid such as blood or normal saline. The return path device provides a return electrical path from the patient's tissues. Both the instrument and the return path device are connected using electrically conductive wires to the source of the radio frequency electrical energy which serves as both the source and the sink for the electrical energy to produce a complete electrical circuit. When the instrument and the return path device are separate devices the technique is termed monopolar. In some cases the instrument contains working surfaces that both supply the electrical energy and provide the return path. In these cases the technique is termed bipolar.
FIG. 3 illustrates a schematic of an electrosurgical system generally of the above-described type which includes an electrosurgical generator 1 with the generator electronics 2 (including the radio frequency (RF) electrical energy source, controls, and power supply being included in the electronics), as well as an electrosurgical accessory or instrument 100 and a return system 110 which is mechanically separated from the accessory 100. As such, the configuration of FIG. 3 is of the monopolar type. An output connector plug 3 and a return connector plug 4 of the accessory 100 connect to the output connector 5 and the return connector 6 that are part of the generator 1. The output connector plug 3 and a return connector plug 4 typically are molded plastic parts with metallic prongs (not shown) or receptacles (not shown). One or more of the metallic prongs in the output connector plug 3 connect to the output line 7 of the accessory 100, which typically consists of one or more conductive metal wires covered with an insulating coating. The output line 7 passes from the distal end of the output connector plug 3 and has a length suited to have the handle 8 of the accessory 100 a comfortable distance from the generator 1. The output line 7 passes into the proximal end of the accessory handle 8. The output line 7 is routed through the accessory handle 8 and may connect to a variety of internal conductors (not shown) that eventually make electrical contact with the active element 9 of the accessory 100, such as a blade. The accessory active element 9 may be in either direct or indirect contact with the patient 10. Electrosurgical energy passes from the active element 9 to the patient 10. The electrical return path is provided by the return system 110, which again is separate from the accessory 100 in the illustrated monopolar configuration of FIG. 3. The return system 110 consists of the return line 11 which typically connects with one or more metallic receptacles (not shown) that are molded into the housing of the return connector plug 4 and that, in turn, connect to the return connector 6 that is part of the generator 1. The return line 11 typically consists of one or more conductive metal wires covered with an insulated coating. The return line 11 exits the distal end of the return connector plug 4 and connects to the return path device 12 of the return system 110, which is usually a return electrode pad when monopolar procedures are used and as contemplated by the configuration of FIG. 3.
A variation of the accessory 100 from FIG. 3 is presented in FIG. 4 in the form of a schematic of an electrosurgical accessory 100′. In this case a supplemental return line 13 of the return system 110′ extends from the return connector plug 4 to the output connector plug 3 where it interfaces with the return line 11. The supplemental return line 13 will be long enough to span the distance between the output connector 5 and the return connector 6 and allow the user enough slack to conveniently connect the output connector plug 3 and the return connector plug 4 to the generator 1. This length will typically be between 6 and 18 inches. The length should not be longer than necessary to avoid producing confusing clutter.
The output line 7 and the return line 11 may leave the output plug 3 separately or joined together in a cable in the case of either of the configurations presented in FIGS. 3–4. Although this is appropriate for the monopolar configurations presented in FIGS. 3–4, joining the lines together is particularly advantageous when they both go to an accessory which is of the bipolar type, and one embodiment of which is schematically presented in FIG. 5. In this case, the accessory handle 8 of the accessory 150 provides electrical continuity from both the output line 7 and the return line 11 to the active element 9 and the return path device 12 (e.g., return electrode), respectively. In bipolar accessories in general, the active element 9 and the return path device 12 are often joined together mechanically, but not electrically, using an accessory electrode housing 14. The accessory electrode housing 14 can be of many forms, of which an insulated shaft is an example. The common feature of the various forms of the accessory electrode housing 14 is that it allows both the active element 9 and the return path device 12 to contact simultaneously the patient 10. Such contact may be either direct or indirect.
One embodiment of a prior art bipolar configuration is more particularly illustrated in FIG. 15, which is used in conductive liquid environments. The accessory 200 operatively interfaces with an electrosurgical generator (not shown) via an output connector 5 on the generator and a return connector 6 on the generator. The accessory 200 has a supplemental return line 13 passing from the return connector 4 to the output connector 3. The accessory 200 illustrated in FIG. 15 is a bipolar electrosurgical accessory that uses a return electrode and it will be compared to later figures to illustrate distinctive features of the subject invention. The device 200 illustrated in FIG. 15 includes a probe assembly 27 that has a probe handle 28 and a probe shaft 29. The output line 7 and the return line 11 are of a length needed to allow the surgeon to conveniently place the electrosurgical generator. The probe shaft 29 is coated with probe shaft insulation 30 that extends almost the complete length of the probe shaft 29. The probe shaft 29 is typically made of either a polymer, which may be flexible, or, more commonly, of metal. One or more channels (not shown) may pass through the probe shaft 29 to allow irrigation solution, aspirated materials, tools, light sources, or visualization equipment to pass into the patient. At the distal tip of the probe shaft 29 is the active electrode assembly 31 which includes the active electrode 32. The output line 7 may continue through the length of the probe assembly 27 and electrically connect to the active electrode 32. If the output line 7 does not directly connect to the active electrode, then one or more conductive elements (not shown) form a conductive path to the active electrode 32. The probe shaft 29 is electrically connected to the return line 11. A section of the probe shaft 29 is left uninsulated to be the return electrode 33. The illustrated device shows a probe shaft 29 made only from metal. If a polymeric or other insulating material forms the probe shaft 29, then the shaft 29 is not insulated and a conductive metal element is attached to form the return electrode 33. The active electrode assembly 31 is insulated from the return electrode 33 by an active electrode standoff insulator 34.
The return electrode 33 is a conductor that contacts whatever liquid (not shown) may be surrounding it. A perforated shield (not shown) may surround the return electrode 33 as well, but the shield allows conductive liquid to contact the return electrode 33. The conductive liquid needs to contact the return electrode 33 to form an electrically conductive path.
The probe shaft insulation 30 is selected to insulate the probe shaft 29 from contacting patient tissues that may lead to inadvertent electrical return paths. The insulation 30 is not selected to allow energy transfer by electrical fields to the probe shaft 29, and such energy transfer is not required, nor can it occur, when the return electrode 33 has electrical continuity with surrounding conductive liquid to generate a current return path.
The waveforms produced by the radio frequency electrical source in an electrosurgical procedure are designed to produce a predetermined effect such as tissue ablation or coagulation when the energy is conveyed to the patient's tissue. The characteristics of the energy applied to the tissue, such as frequency and voltage, are selected to help achieve the desired tissue effect.
Electrosurgical procedures can experience inadvertent problems that lead to unintended tissue damage. During electrosurgical procedures the depth of the effect to the tissue depends upon tissue properties, which change during the application of energy. It is desirable to not have the tissue effects change so rapidly that the surgeon has difficulty controlling the surgical result. During some procedures, such as minimally invasive surgical (MIS) procedures wherein surgical instruments are passed through small openings in the patient's tissue, energy can enter a patient's tissue at a location other than where the active electrode is positioned. Such inadvertent energy application can lead to burns or other complications. When surgical instruments are being inserted or withdrawn from patients during MIS procedures, concern exists for inadvertent activation of the RF energy source and tissue damage that could occur from such an event. One aspect of this problem occurs when the return path device is positioned such that it causes high current flux through tissue adjacent to it. High current flux can cause tissue burns or other damage. It would be desirable for the devices used by surgeons to not allow such inadvertent high current fluxes to occur.
The source of RF energy (the generator) has an output power that depends upon the operating characteristics of its design, including the design of its internal circuitry. Typically the generator is set by the clinical user to a setting that represents the output power desired. When the generator operates, the output power typically depends upon the impedance of the load into which the generator is delivering power. In general, the various generators available operate in modes that approximate constant voltage devices, constant power devices, or some hybrid mode that lies between constant voltage and constant current. The modes approximate constant voltage or constant power output due to the variations inherent in electronic component performance. Modern general purpose generators commonly used in operating rooms typically operate in a constant power mode when power outputs other than low power are desired.
Generator supply companies have long recognized the desirability of using constant power for major surgical procedures such as open surgical procedures. Consequently, the modern generators in operating rooms use a constant power mode. Recently, procedures, such as arthroscopic surgical procedures (e.g., tissue ablation and capsular shrinkage), that benefit from using a constant voltage mode have become increasingly common and important. Special purpose generators have been developed for these constant voltage procedures. Surgical instruments connect to the constant voltage generator and the RF energy is conveyed to the working surfaces using conductors of various types.
Constant power can lead to runaway interactions between the RF energy and the tissue. During electrosurgical procedures the tissue impedance eventually increases as the tissue is affected by the energy imparted to it. In an attempt to continue delivering constant power, a constant power source will increase the output voltage to overcome the increased tissue impedance. This increased voltage will lead to continued changes in the tissue with corresponding increases in tissue impedance, which, in turn, cause the generator to increase the voltage further. The cycle of events usually occurs very rapidly, so rapidly that during some procedures it is beyond the user's ability to respond quickly and prevent undesired tissue effects such as charring or excessive tissue destruction.
Constant voltage automatically reduces the rate that energy is supplied to the tissue as the tissue impedance increases. When constant voltage is used, the current delivered to the tissue, and consequently the power delivered, decreases as tissue impedance increases. This inherent response can greatly reduce or eliminate runaway interactions between the tissue and the RF energy applied to it.
To date, the advantages of constant voltage cannot be easily obtained from constant power generators. It would be beneficial for users, when they so desire, to easily and economically be able to have constant power generators deliver constant voltage to a surgical site. In particular, it would be useful for users to achieve the benefits of constant voltage supply without needing to modify existing generators or attach special adapters to generators. In cases where single use, or limited use, disposable surgical accessories are used, it would be particularly beneficial if the accessory makes the conversion from constant power to constant voltage. For example, it would be beneficial if an arthroscopic instrument intended for ablating tissue could be plugged into a constant power generator and apply power that approximated constant voltage to the tissue.
To date, the primary means for delivering RF energy to tissue while employing constant voltage requires using a constant voltage generator. Constant voltage electrosurgical generator design is known art, such that described in U.S. Pat. No. 5,472,443. Constant voltage electrosurgical generators have outputs that are constant voltage and do not convert the output from a constant power generator to be constant voltage. U.S. Pat. No. 5,472,443 also presents a means for retrofitting selected generators to modify the output, however the circuit presented has considerable complexity and does not lend itself to use in disposable products. The U.S. Pat. No. 5,472,443 circuit is also intended for use with surgical instruments that cut using a sharp edge, rather than using electrosurgical energy to produce the cutting action. Other known electrosurgical generator art limits the current flow, such as described in U.S. Pat. Nos. 4,092,986, 5,267,997 and 5,318,563. The art described in these patents is incorporated into generators and does not convert the mode of a constant power generator into constant voltage. U.S. Pat. Nos. 4,114,623 and 5,891,095 describe current limiting means, as opposed to voltage limiting means. Electrosurgical systems may use temperature sensing to control the power applied to the tissue, such as described in U.S. Pat. No. 5,440,681.